Adding Flash Devices as ZFS Log or Cache Devices

Review the following recommendations when adding flash devices as ZFS log or cache devices.

A ZFS log or cache device can be added to an existing ZFS storage pool by using the zpool add command. Be very careful with zpool add commands. Mistakenly adding a log device as a normal pool device is a mistake that will require you to destroy and restore the pool from scratch. Individual log devices themselves can be removed from a pool.
Familiarize yourself with the zpool add command before attempting this operation on active storage. You can use the zpool add -n option to preview the configuration without creating the configuration. For example, the following incorrect zpool add preview syntax attempts to add a device as a log device:
```
# zpool add -n tank c4t1d0
vdev verification failed: use -f to override the following errors:
mismatched replication level: pool uses mirror and new vdev is disk
Unable to build pool from specified devices: invalid vdev configuration
```
This is the correct zpool add preview syntax for adding a log device to an existing pool:
```
# zpool add -n tank log c4t1d0
would update 'tank' to the following configuration:
tank
mirror
c4t0d0
c5t0d0
logs
c4t1d0
```
If multiple devices are specified, they are striped together. For more information, see the examples below or the zpool(8) man page.

A flash device, c4t1d0, can be added as a ZFS log device:

# zpool add pool log c4t1d0

If 2 flash devices are available, you can add mirrored log devices:

# zpool add pool log mirror c4t1d0 c4t2d0

Available flash devices can be added as a cache device for reads.

# zpool add pool cache c4t3d0

You can't mirror cache devices, they will be striped together.

# zpool add pool cache c4t3d0 c4t4d0